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Hunting for exploding stars - and the nature of dark energy

Astronomers using the Sloan Digital Sky Survey (SDSS) telescope at Apache Point Observatory spent three months last fall scanning the sky for supernovae - exploding stars in distant galaxies - and the hunting was good.

NMSU Astronomer Jon Holtzman is part of a team of scientists using the Sloan Digital Sky Survey telescope to locate supernovae - exploding stars. (NMSU photo by Darren Phillips)

"The survey was very successful," said New Mexico State University astronomer Jon Holtzman, a member of the SDSS team. "In 90 days we identified a large number of supernova candidates, and with follow-up observations we confirmed about 130 of them to be Type Ia supernovae."

The large sample, and the specific type of supernova, are important because astronomers can use the supernovae as "standard candles" or distance signposts to help them calculate the rate of expansion of the universe. And that will lead them to a better understanding of the mysterious dark energy that is believed to be causing the expansion to accelerate.

Supernovae occur when stars explode at the end of their life cycles, releasing huge amounts of energy, gas and particles. Because they are so bright - one supernova can be as bright as the billions of other stars in its galaxy combined - they can be seen over vast distances, Holtzman said. They fade in brightness over the course of several weeks, however, so astronomers must work quickly to identify supernova candidates and do follow-up spectroscopy before they fade.

Just as there are different types of stars there are different types of supernovae. Type Ia ("one A") supernovae, which occur in binary or twin-star systems, are useful because they all reach about the same level of brightness, Holtzman said.

"Knowing their intrinsic brightness, you can measure how far away they are," he said.

The Sloan supernova survey, a three-year campaign that will continue during the fall months over the next two years, will help scientists more accurately measure the expansion of the universe by providing a large sample of supernovae over a range of distances that has not been searched well before now.

"In the past, supernovae have been found in two ways," Holtzman said. "They have been found nearby using smaller telescopes and looking at nearby galaxies. Very distant ones have been found using big telescopes and looking at a smaller piece of the sky. There haven't been many done in the intermediate range."

The Sloan Digital Sky Survey's 2.5-meter telescope scans large segments of the sky, making it ideal for bridging this distance gap, he said. With a large number of Type Ia supernovae to use as distance guideposts in this middle range, scientists will be able to probe the cosmic expansion and the nature of dark energy with greater precision.

Follow-up spectroscopy of the supernova candidates found by the Sloan survey was done by telescopes all over the world, including the 3.5-meter Astrophysical Research Consortium (ARC) telescope at Apache Point Observatory, located high in the Sacramento Mountains near Cloudcroft.

NMSU is a member of the ARC, shares in the telescope's time and operates the observatory for the consortium. Besides the 3.5-meter ARC telescope and the 2.5-meter Sloan telescope, the observatory is home to NMSU's 1-meter telescope, which was used to back up the Sloan instrument in the supernova survey.

With a large sample of Type Ia supernovae to work with, Holtzman said, "now we need to measure their brightness very accurately." He and his colleagues hope to have results to report in the summer.

The Sloan Digital Sky Survey is an international collaboration to map one-quarter of the entire sky, determining the positions and absolute brightnesses of more than 100 million celestial objects.

SDSS completed most of its primary survey during its initial five-year period. The supernova survey is one of three major projects being undertaken in a three-year extension known as SDSS-II. Funding is provided by the Alfred P. Sloan Foundation, the participating institutions, the National Science Foundation, the U.S. Department of Energy, NASA, the Japanese Monbukagakusho, the Max Planck Society and the Higher Education Funding Council for England.